It is known that (−)-3a,6,6,9a-tetramethyl dodecahydronaphtho[2,1-b]furan represented by the following general formula (I) and optical isomers thereof (hereinafter collectively referred to as merely “(±)-ambroxan”):
are important amber-like perfume materials having an excellent aromatizing property and an excellent aroma retention property.
The (±)-ambroxan contains a plurality of asymmetric carbons. Therefore, there are present diastereomers of the (±)-ambroxan. There is conventionally known a process for producing a mixture of the diastereomers of the (±)-ambroxan (hereinafter referred to merely as “(±)-ambroxans”) represented by the following general formula (II):
in which a homofarnesylic acid amide or a monocyclohomofarnesylic acid amide is cyclized in the presence of an acid agent and then hydrolyzed, and further the obtained hydrolyzed product is subjected to reduction reaction and cyclization reaction (refer to Patent Document 1).    Patent Document 1: Japanese Patent Application Laid-Open (kokai) No. 2008-56663